There are a few conversions like this where I live (also in the UK) and I agree that the results definitely do vary. Where the conversion is in the heart of the city, it seems to work pretty well -- sometimes even better than knocking down and rebuilding, because the existing offices usually have stand-off space around the building that would be lost in a high density redevelopment. And from an environmental perspective, keeping all of that 1950s/1960s poured concrete in place has got to be a good thing.
What really doesn't work is the partial conversion of offices on industrial/commercial parks out of town. It seems to lead to completely incongruous buildings, dumped essentially in the middle of parking lots, without basic amenities such as verges and pavements. Those buildings tend to be flimsy, and it's hard to see the environmental argument for keeping them. I would rather see those sites razed and rebuilt as better quality housing.
What really doesn't work is the partial conversion of offices on industrial/commercial parks out of town. It seems to lead to completely incongruous buildings, dumped essentially in the middle of parking lots, without basic amenities such as verges and pavements. Those buildings tend to be flimsy, and it's hard to see the environmental argument for keeping them. I would rather see those sites razed and rebuilt as better quality housing.